Purchasing specifications for vinyl chloride commonly call for monovinyl acetylene ("MVA") and butadiene ("BDE") in amounts, for example, of less than ten parts per million because these impurities have been found to cause difficulties in the polymerization or copolymerization of the vinyl chloride and/or in the polymeric products made in their presence. Thus it is necessary for producers of vinyl chloride to use manufacturing processes which will not generate the undesired amounts of the impurities, and/or to develop methods of removing or reducing the concentration to acceptable levels, which is the subject of the present invention.
U.S. Pat. No. 3,142,709 discloses a method of purifying vinyl chloride containing butadiene as an impurity, wherein the vinyl chloride is treated with anhydrous hydrogen chloride. Contacting vinyl chloride contaminated with butadiene and monovinyl acetylene with catalytic amounts of Lewis acids such as AlCl.sub.3 and FeCl.sub.3 was disclosed in U.S. Pat. No. 3,723,550. Products of this process are described as heavies which can be removed relatively easily by distillation or other conventional means, in contrast to the monomeric butadiene and other C.sub.4 's which are extremely difficult to remove by distillation. In U.S. Pat. No. 4,760,206 which discloses reacting the butadiene present in a vinyl chloride stream with chlorine in the presence of ferric chloride, the ferric chloride is said to polymerize butadiene in vinyl chloride (column 2, lines 20-30); however, if the ferric chloride itself becomes a contaminant in the vinyl chloride, it will interfere with the polymerization of the vinyl chloride (column 1, lines 25-26). Molecular chlorine is also discussed as a decontaminant in U.S. Pat. No. 3,125,607 (see especially Example 2); U.S. Pat. Nos. 3,876,714; and 3,920,761.
Difficulties encountered in attempts to design an improved method of removing the impurities butadiene and/or monovinyl acetylene include not only the known tendency of ferric chloride to be carried over in the treated stream, but also that vinyl chloride can react with hydrogen chloride to produce 1,1-dichloroethane, another compound not wanted in the purified vinyl chloride.